Lattice-mismatched materials find use in a variety of devices employing strained layers, such as opticallyaddressed spatial light modulators, high electron mobility transistors which operate at room temperature, infrared sensors, and lasers. Lattice-mismatched materials show different growth modes as compared to such lattice-matched materials as GaAs/AlGaAs. Thin films of such materials evidence extensive dislocations and a very rough surface morphology if grown continuously, as is current conventional practice.
In a particular example, it is desired to achieve the growth of high quality (low defect density) InAs strained layer films on GaAs (100) substrates. Films are required which are ten to thirty monolayers thick for certain device applications, but existing technologies give only one monolayer, before generation of extensive dislocations.
The growth of lattice-mismatched materials is extremely difficult because the system wants to relieve the developing lattice strain (in the course of film growth) by spontaneously injecting misfit dislocations. These dislocations are generally accompanied by other point defects and are electrically active. The resulting material has very poor electrical properties (low carrier lifetime, Fermi level pinning). This is the general situation obtained in classical MBE (molecular beam epitaxial) growth of InAs films on GaAs (100) surfaces. These films have a lattice mismatch of more than 7.4% (InAs&gt;GaAs). Classical MBE growth refers to the continuous deposition of elemental In on a GaAs surface which is flooded by an excess As.sub.4 beam. The elemental In and As react together on the surface at elevated temperatures (&gt;200.degree. C.) to form the InAs compound.